Titanium alloys are frequently used in aerospace and aeronautical applications because of their high strength, low density, and corrosion resistance. Although pure titanium has desirable properties for many uses, it is often unsuitable for more demanding structural applications. To achieve the necessary strength and fatigue resistance for use in most aerospace and aeronautical applications, titanium is typically alloyed with other elements. Two prevalent titanium alloys in use in aerospace and aeronautical applications are Ti 64 and Ti 6242. Both of these alloys are titanium-based alloys, meaning that titanium makes up the majority of the alloy in terms of weight percentage. Ti 64 is an alpha-beta alloy that has nominal elemental compositions of about 6 weight percent (wt. %) aluminum and 4 wt. % vanadium, with the balance being titanium. Ti 6242 is also an alpha-beta alloy that has nominal elemental compositions of about 6 wt. % aluminum, 2 wt. % tin, 4 wt. % zirconium, and 2 wt. % molybdenum, with the balance being titanium. Another recently developed titanium alloy that is useful in aerospace and aeronautical applications is Ti 5553 that has nominal elemental compositions of about 5 wt. % aluminum, 5 wt. % vanadium, 5 wt. % molybdenum, 3 wt. % chromium, 0.5 wt. % iron, and 0.15 wt. % oxygen, with the balance being titanium.
Titanium and titanium-based alloys typically exhibit two-phase microstructures. Pure titanium exists as alpha phase having a hexagonal close-packed crystal structure up to its beta transus temperature (about 885° C.). Above the beta transus temperature, the microstructure changes to the beta phase, which has a body-centered-cubic crystal structure. Certain alloying elements may be added to control the microstructure and thereby allow the beta phase to be at least metastable at room temperature. Alpha-beta alloys are typically made by adding one or more beta stabilizers, such as vanadium, which inhibit the transformation from beta phase back to alpha phase and allow the alloy to exist in a two-phase alpha-beta form at room temperature.
Titanium alloys are typically more difficult to machine than many other common aerospace materials such as aluminum-based alloys. Furthermore, some titanium alloys are significantly more difficult than other titanium alloys to machine, with machinability being directly associated with their alloy phase compositions. When examined by alloy phase it is generally understood that despite their higher strength, beta titanium alloys are more difficult to machine than alpha or alpha-beta titanium alloys. This relationship between crystal structure phase and machinability is somewhat problematic for some common titanium alloy processing procedures, which include a beta annealing step immediately after forging the alloy in order to provide increased strength to the part that is formed from the alloy. A particular processing method performed after forging a titanium alloy and before machining the alloy is referred in the art as a “BASCA” process, which includes beta annealing, followed by slow cooling and aging the titanium alloy. Although the BASCA process provides superior strength to many alloys, tests reveal that carbide cutters lose a substantial amount of useful life when machining titanium alloys that are almost entirely beta phase. For example, uncoated carbide cutters used for machining BASCA-treated Ti 5553 alloy have about 25% of their ordinary useful life when compared with Ti6Al4V alloy in a mill-annealed condition having a small beta phase concentration.
There is no conventional process for effectively improving the intrinsic machinability of titanium alloys or other alloys. Some composition-related approaches have typically included adding specific elements to an alloy to change its machining behavior. For example, elements such as sulfur are commonly added to a stainless steel alloy to improve its machinability. However, these chemical treatments typically are performed at the expense of at least some mechanical properties for the treated alloy. Other approaches have included adjusting machining parameters such as cutting tool speeds and characteristics, alloy feed rates, or coolant levels. Although cutting tools and machining processes continue to improve, no tool or process has been universally identified to be exceptionally effective for machining titanium alloys.
Accordingly, it is desirable to improve the machinability of titanium alloys. In addition, it is desirable to provide titanium alloys and parts made therefrom having superior strength without sacrificing machinability. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background.